Heat olive oil in a large saucepan over medium-high heat, add onion and stir occasionally until golden (4-5 minutes). Add carrot and celery and stir occasionally until softened slightly (3-4 minutes), add stock, potato, ribs, lentils and thyme, bring to the boil, then reduce heat and simmer until lentils are tender and potato begins to break down (45 minutes-1 hour). Season to taste and serve drizzled with extra-virgin olive oil and crusty bread.